1. Field of the Invention
The present invention relates to a microwave oven for cooking food placed on a rotating tray by using a microwave, and more particularly, to a tray driving apparatus for a microwave oven and a microwave oven having the same for enhancing the cooking quality by periodically changing the rotational speed of the tray to much more evenly radiate a microwave  microwaves to the food.
2. Description of the Prior Art
A microwave oven is a device for cooking food by using a microwave  microwaves generated from a magnetron. The principle is that the microwave cooks  microwaves cook food from its interior by heating molecules of water included in the food. The microwave oven is more widely used since the microwave oven has clear advantages in a higher thermal efficiency, more rapid heating, and less loss of nutrients.
Such a microwave oven, as shown in FIG. 1, has a cooking chamber 12 and a driving chamber 14 in the body 10. The cooking chamber 12 is a portion in which food is placed and cooked and on the front side of which a door is mounted to be opened and closed. A tray 16 for placing food is mounted on the bottom of the cooking chamber 12. The driving chamber 14 has various electric components and parts such a  as magnetron 17, a high voltage transformer 18, a waveguide, a cooling fan 19, and a control part to generate and radiate a microwave  microwaves into the cooking chamber 12. A control panel 30 is prepared on the front side of the driving chamber 14 in order for a user to set various cooking modes and to operate the microwave oven.
If the electric components in the driving chamber 14 are operated, a microwave  microwaves generated in the magnetron 17 is  are guided to the cooking chamber 12 through the wavelength 17. The microwave  microwaves guided in the cooking chamber 12 is  are directly radiated to food or indirectly radiated to food while being reflected from the walls of the cooking chamber 12. The microwave radiated to food generates  microwaves radiated to the food generate heat by vibrating water molecules of food. Then food is cooked by the generated heat.
In the meantime, in general, according to the shape of the cooking chamber 12, and the shape and size of food placed on the tray 16, the microwave distribution in the interior of the cooking chamber 12 changes, as well as continuously changing with respect to time. According to this, since the microwave is  microwaves are not evenly radiated to the entire food, the respective portions of food are differently cooked, thus the cooking quality is degraded.
In order to solve such a phenomenon, a general microwave oven has a tray driving apparatus for rotating the tray 16. It is experientially known that a microwave is  microwaves are more evenly radiated to food to enhance the cooking quality if food placed on the tray 16 is rotated by the tray driving apparatus.
Such a tray driving apparatus, as shown in FIG. 2, has a driving source 42, and a driving shaft 44 for transmitting the driving force to the tray 16. A general motor is used for a driving source, and a plurality of gears are included for reducing the number of rotations of the motor which is rotated at a high speed, and for transmitting a driving force to the driving shaft 44.
If cooking starts, the driving source 42 generates a driving force, and the driving force is transmitted to the tray 16 through the driving shaft to slowly rotate the tray 16. According to this, food placed on the tray 16 is rotated at a predetermined speed, and the microwave is  microwaves are evenly radiated to food. Reference numeral 13 in FIG. 2 denotes the bottom panel of the body 10 which forms the bottom portion of the cooking chamber, and reference numeral 46  42 is a connection member  which connects a shaft 44 with respect to the tray (or called  also called a rotary tray) 16.
In the meantime, if the tray is rotated at a constant speed, in the case that a  microwave distribution with respect to time in the interior of the cooking chamber 12 changes according to the rotational speed of the tray 16, the respective portions of food are differently cooked due to the uneven radiation of the microwave to  microwaves to the food even though the tray 16 is rotated. Accordingly, the cooking quality is deteriorated.